1. Field of the Invention
The present invention relates to a refrigerating system and, in particular, to a compressor driving system for use in a vehicle with an engine, such as a refrigerated van or truck.
2. Description of the Prior Art
In order to maintain food, drinks, and other perishables at a proper temperature during transportation, a refrigerating system is often provided in a vehicle such as a van or truck. These vehicles are known in the market as refrigerated vans or chill cars.
FIG. 1 shows a schematic block diagram of a refrigerating system typical of the prior art systems. The refrigerating system 100 includes refrigerant circuit 200 comprising first compressor 201 for compressing refrigerant, condenser 202, receiver-drier 203, expansion valve 204, evaporator 205 for cooling a refrigerated compartment (not shown) and pipe members 206 for connecting the outlet of each of the elements of the refrigerant circuit to the inlet of each adjacent elements of the refrigerant circuit. First compressor 201 is provided with electromagnetic clutch 211 for intermittently receiving the driving force from vehicle engine 101 through belt 102. The intermittent operation of the electromagnetic clutch 211 serves to intermittently couple the engine to the compressor. Thus, first compressor 201 is intermittently driven by engine 101 through electromagnetic clutch 211.
Refrigerating system 100 further comprises a standby unit 110 including second compressor 111 and three-phase motor 112 for driving second compressor 111 through belt 113. Outlet port 111a of second compressor 111 is connected to the pipe member between first compressor 201 and condenser 202 through pipe member 206. Inlet port 111b of second compressor 111 is connected to the pipe member between evaporator 205 and first compressor 201 through pipe member 206. Three-phase motor 112 may be directly connected to a commercial AC power source 114 through power source box 115. Power source box 115 also functions as both a rectifier and transformer in order to supply electric power to other electric equipment incorporated in the refrigerant circuit. This equipment may include, for example, an evaporator fan motor (not shown).
Second compressor 111 is provided with electromagnetic clutch 116 for intermittently receiving the driving force from motor 112 through belt 113 by virture of the intermittent operation of electromagnetic clutch 116. Thus, second compressor 111 is intermittently driven by motor 112 through electromagnetic clutch 116.
The temperature of the refrigerated compartment is dependent on the operation of compressor 201 or 111. Therefore, temperature control in the refrigerated compartment is achieved by intermittent operation of electromagnetic clutch 211 or 116, which operate compressors 201 and 111, respectively.
When the vehicle engine is operating, first compressor 201 is driven by the vehicle engine to circulate the refrigerant through refrigerant circuit 200. On the other hand, when the vehicle engine is stopped, such as when the driver is sleeping through the night, second compressor 111 is driven by motor 112. Compressor 201 does not operate since its power source, the engine, is stopped.
In the prior art, the additional provision of second compressor 111 of standby unit 110 complicates piping of the refrigerant circuit.
Furthermore, it is difficult to carry out accurate temperature control in the refrigerated compartment because the refrigerant compressor is driven by the engine which often varies in rotational speed as the vehicle is driven.
To eliminate these drawbacks, another refrigerating system has been proposed. The refrigerating system comprises a refrigerant circuit including a compressor having a three-phase motor and a compressing mechanism driven by the three-phase motor. The motor and compressing mechanism are hermetically contained in one casing. In this refrigerating system, when the vehicle engine is running, the motor of the compressor is connected to a generator driven by the vehicle engine. The generator provides the necessary power to the motor. On the other hand, when the vehicle engine is stopped, the motor driving the compressor is connected to a commercial AC electric power source instead of the vehicle generator. Thus, the motor receives its power from a commercial AC power source. The motor, as it receives power, drives the compressor of the refrigerating system providing the needed cooling.
The temperature in the refrigerated compartment is controlled by switching on and off the generator or by intermittently connecting the commercial AC electric power source to the motor. That is, temperature in the refrigerated compartment is controlled by the intermittent operation of the motor, the rotational speed of which is maintained at a certain value.
However, since the temperature in the refrigerated compartment is controlled by the intermittent operation of the motor, it is still difficult to carry out accurate temperature control in the refrigerated compartment.